1. Field of the Invention
This invention relates generally to processes for depositing titanium-containing oxide films. Certain embodiments relate to processes for manufacturing titanium-containing oxide thin films by atomic layer deposition using volatile titanium compounds as source materials.
2. Description of the Related Art
Atomic layer deposition (“ALD”) refers to vapour deposition-type methods in which a material, typically a thin film, is deposited on a substrate from vapour phase reactants. It is based on sequential self-saturating surface reactions. ALD is described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811, incorporated herein by reference. ALD reactors benefit from the use of inert carrier and purging gases, which makes the system fast.
According to the principles of ALD, the reactants (also referred to as “source chemicals” or “precursors”) are separated from each other, typically by inert gas, to prevent gas-phase reactions and to enable the above-mentioned self-saturating surface reactions. Surplus chemicals and reaction by-products are removed from the reaction chamber by purging with an inert gas and/or evacuating the chamber before the next reactive chemical pulse is introduced. Undesired gaseous molecules can be effectively expelled from the reaction chamber by keeping the gas flow speeds high with the help of an inert purging gas. The purging gas pushes the extra molecules towards the vacuum pump used for maintaining a suitable pressure in the reaction chamber. ALD provides controlled film growth as well as outstanding conformality.
Titanium containing oxides are technologically very important and they have a variety of industrially useful properties. They function well, for example, as semiconductors, insulators and ferroelectrics. TiO2 has a high permittivity of around 70. BaTiO3 and SrTiO3 have permittivities of several hundreds. Titanium also has several technologically important ternary compounds, such as BiTiO3 and PbTiO3. Pure TiO2 is usually oxygen deficient and thus semiconducting. Semiconducting TiO2 has been used, for example, in solar cells and self-cleaning coatings.
Alkaline earth metals (such as Ba and Sr) easily form stable non-volatile halides. Therefore, halide-containing precursors of these metals are not generally useful in depositing oxides such as SrTiO3 and BaTiO3 by ALD. In addition, alkaline earth metals easily form hydroxides. As a result, ALD using water as an oxygen source can be problematic, requiring long purge times and/or high temperatures. However, long purge times effectively impair the productivity of these processes. Further, it can be difficult to find a deposition temperature that will not cause decomposition of the precursors and will keep the thin film atoms intact, but will still keep the precursors in gaseous phase and provide the activation energy for the surface reactions.